Our eldest, Malcolm, whom we name hereafter
The Prince of Cumberland; which honour must
Not unaccompanied invest him only,
But signs of nobleness, like stars, shall shine
On all deservers. (1.4.45-9)

Here Duncan is settling matters of state. He has happily made Macbeth Thane of Cawdor and now he chooses his son to become the Prince of Cumberland, next in line to his throne (in medieval Scotland the son did not automatically become heir). Duncan adds that this honour of a higher title must not adorn ('invest') only Malcolm; other men who have served him well will also receive new and stately titles. Unfortunately, bestowing this honour upon his son, and not Macbeth, costs Duncan his life in the next scene.